The use of ultrasound for imaging and diagnosis of disease is well known in the medical field. In therapeutic applications, absorbed ultrasound energy is used to change the state of a target area. In particular, ultrasound energy applied at high power densities can induce significant physiological effects on tissues. These effects may result from either thermal or mechanical response of the tissue subjected to ultrasound energy. Thermal effects include hyperthermia and ablation of tissue. The absorption of ultrasound energy at the target area induces a sudden temperature rise, which causes coagulation or ablation of target area cells.
In therapeutic applications of ultrasound, it is important that the applied ultrasound energy causes an intended change of state solely at a target area without adversely affecting other tissue within the patient. The effective therapeutic dose must be delivered to the target area while the thermal and mechanical effects in intermediary and surrounding tissue are minimized. Therefore, proper focusing and control of HIFU is one of the primary criteria for successful therapeutic application of ultrasound.
As described in U.S. Pat. No. 6,007,499 to Martin et al. and U.S. Pat. No. 6,042,556 to Beach et al., which are incorporated herein by reference, in HIFU hyperthermia treatments, the intensity of ultrasonic waves generated by a focused transducer increases from the source to the region of focus, where it can reach a very high temperature. The absorption of the ultrasonic energy at the focal region induces a sudden temperature rise of affected tissue that causes the irreversible ablation of the target volume of cells. HIFU hyperthermia treatments may be intended, for example, to cause necrosis of an internal lesion without damage to intermediate tissues.
Methods have been developed to increase the intensity of HIFU used for therapeutic purposes. For example, U.S. Pat. No. 5,092,336 to Fink, which is incorporated herein by reference, describes a device for localization and focusing of acoustic waves in tissues. The invention is based upon a technique known as time-reversed acoustics, which is described in an article by Fink, entitled, “Time-reversed acoustics,” Scientific American, November 1999, pp. 91–97, which is also incorporated herein by reference. Essentially, a target is enclosed by an array of transducers that delivers an unfocused acoustic beam on a reflective target in a medium, for example, a site in organic tissue. Reflected signals from the target detected by ultrasound transducers in a regular array outside the patient are stored, the distribution in time and the shapes of the echo signals are time-reversed, and the reversed signals are applied to the respective transducers of the array. In most cases, the target constitutes a secondary source, which reflects or scatters a wave beam applied to it.
U.S. Pat. No. 6,161,434 to Fink et al., which is incorporated herein by reference, describes methods to use time-reversed acoustics to search for a faint sound source. U.S. Pat. No. 5,428,999 to Fink, which is also incorporated herein by reference, describes methods for detecting and locating reflecting targets, ultrasound echographic imaging, and concentrating acoustic energy on a target.
One of the areas where ablation of tissue has proven to be an effective therapeutic technique is the treatment of cardiac arrhythmia, particularly atrial fibrillation. Cardiac arrhythmia occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thus disrupting the normal cycle and causing asynchronous rhythm. It has been found that primary sources of undesired signals are located in the tissue region along the pulmonary veins of the left atrium and in the superior pulmonary veins. After unwanted contractions are generated in the pulmonary veins or conducted through the pulmonary veins from other sources, they are conducted into the left atrium where they can initiate or continue atrial fibrillation. By selective ablation of cardiac tissue, it is possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process essentially destroys the unwanted electrical pathways by formation of non-conducting lesions.
Various means for performing ablation of cardiac tissue in the prior art include injection of a chemical such as ethanol into specific regions of the heart, or application of cryoablation, RF electrical current, microwave energy, lasers, and ultrasonic energy.
U.S. Pat. No. 5,807,395 to Mulier et al., and U.S. Pat. No. 6,190,382 to Ormsby et al., which are incorporated herein by reference, describe systems for ablating body tissue using radio frequency. U.S. Pat. Nos. 6,251,109 and 6,090,084 to Hassett et al., U.S. Pat. No. 6,117,101 to Diederich et al., U.S. Pat. Nos. 5,938,660 and 6,235,025 to Swartz et al., U.S. Pat. Nos. 6,245,064 and 6,024,740 to Lesh et al., U.S. Pat. Nos. 6,012,457, 6,164,283, 6,305,378 and 5,971,983 to Lesh, U.S. Pat. No. 6,004,269 to Crowley et al., and U.S. Pat. No. 6,064,902 to Haissaguerre et al., which are incorporated herein by reference, describe apparatus for tissue ablation to treat atrial arrhythmia, primarily by ablating tissue located within the pulmonary veins or on the ostia of the pulmonary veins.
HIFU has been used to ablate tissue within a beating heart. An article entitled “Extracardiac ablation of the canine atrioventricular junction by use of high-intensity focused ultrasound,” by S A Strickberger et al., Circulation, 100, 203–208 (1999), which is incorporated herein by reference, describes the experimental use of HIFU to ablate the atrioventricular junction within a beating heart.
PCT Patent Publication WO 97/29699 to Ben-Haim, entitled, “Intrabody energy focusing,” which is assigned to the assignee of the present patent application and is incorporated herein by reference, describes methods for optimizing irradiation of a target area of the body by using a radiation-sensing probe inserted into the body.
U.S. Pat. No. 5,590,657 to Cain et al., which is incorporated herein by reference, describes a HIFU system including a phased array of ultrasound transducers located outside the patient. Methods for refocusing the beam are described.
U.S. Pat. No. 6,128,958 to Cain, which is incorporated herein by reference, describes an architecture for driving an ultrasound phased array.
U.S. Pat. No. 5,769,790 to Watkins et al., which is incorporated herein by reference, describes a system for combining ultrasound therapy and imaging.
U.S. Pat. No. 5,762,066 to Law et al., which is incorporated herein by reference, describes a HIFU system consisting of an intracavity probe having two active ultrasound radiating surfaces with different focal geometries.
U.S. Pat. No. 5,366,490 to Edwards et al., which is incorporated herein by reference, describes a method for applying destructive energy to a target tissue using a catheter.
U.S. Pat. Nos. 5,207,214 and 5,613,940 to Romano, which are incorporated herein by reference, describe an array of reciprocal transducers which are intended to focus intense sound energy without causing extraneous tissue damage.
U.S. Pat. No. 5,241,962 to Iwama, which is incorporated herein by reference, describes the use of ultrasonic pulses and echo signals to disintegrate a calculus.
An article entitled, “High intensity focused ultrasound effect on cardiac tissues: Potential for clinical application,” by L A Lee et al., Echocardiography, 17(6 Pt 1), 563–566 (2000), which is incorporated herein by reference, describes the use of HIFU to create lesions in mammalian cardiac tissues ex vivo.
An article entitled, “High intensity focused ultrasound phased arrays for thermal ablation of myocardium,” by J U Kluiwstra et al., University of Michigan Medical Center, Department of Internal Medicine (undated), which is incorporated herein by reference, describes the experimental use of a combined ultrasound imaging and therapy system to place lesions at various locations in the heart under real-time ultrasound image guidance.
The following references, which are incorporated herein by reference, may be useful:
Hill C R et al., “Review article: High intensity focused ultrasound-potential for cancer treatment,” Br J Radiol, 68(816), 1296–1303 (1995)
Lin W L et al., “A theoretical study of cylindrical ultrasound transducers for intracavitary hyperthermia,” Int J Radiat Oncol Biol Phys, 46(5), 1329–36 (2000)
Chapelon J Y et al., “New piezoelectric transducers for therapeutic ultrasound,” Ultrasound Med Biol, 26(1), 153–159 (2000)
Chauhan S et al., “A multiple focused probe approach for high intensity focused ultrasound based surgery,” Ultrasonics, 39(1), 33–44 (2001)
Sommer F G et al., “Tissue ablation using an acoustic waveguide for high-intensity focused ultrasound,” Med Phys, 24(4), 537–538 (1997)
Kluiwstra J U A et al., “Ultrasound phased arrays for noninvasive myocardial ablation: initial studies,” IEEE Ultrasonics Symposium Proceedings, Vol. 2, 1604–1608 (1995)
Before performing ablation of cardiac tissue, it is often desirable to construct a map of the cardiac area of interest. Cardiac mapping is used to locate aberrant electrical pathways and currents within the heart, as well as to diagnose mechanical and other aspects of cardiac activity. Various methods and devices have been described for mapping the heart.
U.S. Pat. Nos. 5,546,951 and 6,066,094 to Ben-Haim, and European Patent 0 776 176 to Ben-Haim et al., which are assigned to the assignee of the present patent application and are incorporated herein by reference, describe methods for sensing an electrical property of heart tissue, for example, local activation time, as a function of the precise location within the heart. The data are acquired with a catheter that has electrical and location sensors in its distal tip, and which is advanced into the heart. Techniques for sensing cardiac electrical activity are also described in U.S. Pat. No. 5,471,982 to Edwards et al., commonly-assigned U.S. Pat. Nos. 5,391,199 and 6,066,094 to Ben-Haim, U.S. Pat. No. 6,052,618 to Dahlke et al., and in PCT patent publications WO94/06349 and WO97/24981, which are incorporated herein by reference.
Methods of creating a map of the electrical activity of the heart based on these data are described in U.S. Pat. Nos. 6,226,542 and 6,301,496 to Reisfeld, which are assigned to the assignee of the present patent application and are incorporated herein by reference. As indicated in these patents, location and electrical activity is preferably initially measured on about 10 to about 20 points on the interior surface of the heart. These data points are then generally sufficient to generate a preliminary reconstruction or map of the cardiac surface to a satisfactory quality. The preliminary map is often combined with data taken at additional points in order to generate a more comprehensive map of the heart's electrical activity. In clinical settings, it is not uncommon to accumulate data at 100 or more sites to generate a detailed, comprehensive map of heart chamber electrical activity. The generated detailed map may then serve as the basis for deciding on a therapeutic course of action, for example, tissue ablation, which alters the propagation of the heart's electrical activity and restores normal heart rhythm. Methods for constructing a cardiac map of the heart are also described in U.S. Pat. Nos. 5,391,199 and 6,285,898 to Ben-Haim, and in U.S. Pat. Nos. 6,368,285 and 6,385,476 to Osadchy et al., which are assigned to the assignee of the present patent application and are incorporated herein by reference.
Catheters containing position sensors may be used to determine the trajectory of points on the cardiac surface. These trajectories may be used to infer motion characteristics such as the contractility of the tissue. As described in U.S. Pat. No. 5,738,096 to Ben-Haim, which is assigned to the assignee of the present application and which is incorporated herein by reference, maps depicting such motion characteristics may be constructed when the trajectory information is sampled at a sufficient number of points in the heart.
European Patent Application EP 1 125 549 and corresponding U.S. patent application Ser. No. 09/506,766 to Ben-Haim et al., which are assigned to the assignee of the present patent application and are incorporated herein by reference, describe techniques for rapidly generating an electrical map of a chamber of the heart. The catheter used for these techniques is described as comprising a contact electrode at the distal tip of the catheter and an array of non-contact electrodes on the shaft of the catheter near the distal end. The catheter also comprises at least one position sensor. Information from the non-contact electrodes and contact electrode is used for generating a geometric and electrical map of the cardiac chamber.
U.S. Pat. No. 5,848,972 to Triedman et al., which is incorporated herein by reference, describes a method for endocardial activation mapping using a multi-electrode catheter. A multi-electrode catheter is advanced into a chamber of the heart. Anteroposterior (AP) and lateral fluorograms are obtained to establish the position and orientation of each of the electrodes. Electrograms are recorded from each of the electrodes in contact with the cardiac surface relative to a temporal reference such as the onset of the P-wave in sinus rhythm from a body surface ECG. After the initial electrograms are recorded, the catheter is repositioned, and fluorograms and electrograms are once again recorded. An electrical map is then constructed from the above information.
U.S. Pat. No. 4,649,924 to Taccardi, which is incorporated herein by reference, describes a method for the detection of intracardiac electrical potential fields. The '924 patent is illustrative of non-contact methods that have been proposed to simultaneously acquire a large amount of cardiac electrical information. In the method of the '924 patent, a catheter having a distal end portion is provided with a series of sensor electrodes distributed over its surface and connected to insulated electrical conductors for connection to signal sensing and processing means. The size and shape of the end portion are such that the electrodes are spaced substantially away from the wall of the cardiac chamber. The method of the '924 patent is said to detect the intracardiac potential fields in only a single cardiac beat. The sensor electrodes are preferably distributed on a series of circumferences lying in planes spaced from each other. These planes are perpendicular to the major axis of the end portion of the catheter. At least two additional electrodes are provided adjacent the ends of the major axis of the end portion. The '924 patent describes a single exemplary embodiment in which the catheter comprises four circumferences with eight electrodes spaced equiangularly on each circumference. Thus, in that exemplary embodiment, the catheter comprises at least 34 electrodes (32 circumferential and 2 end electrodes).
PCT application WO 99/06112 to Rudy, which is incorporated herein by reference, describes an electrophysiological cardiac mapping system and method based on a non-contact, non-expanded multi-electrode catheter. Electrograms are obtained with catheters having from 42 to 122 electrodes. The relative geometry of the probe and the endocardium must be obtained via an independent imaging modality such as transesophageal echocardiography. After the independent imaging, non-contact electrodes are used to measure cardiac surface potentials and construct maps therefrom.
U.S. Pat. No. 5,297,549 to Beatty et al., which is incorporated herein by reference, describes a method and apparatus for mapping the electrical potential distribution of a heart chamber. An intra-cardiac multielectrode mapping catheter assembly is inserted into the heart. The mapping catheter assembly includes a multi-electrode array with an integral reference electrode, or, preferably, a companion reference catheter. In use, the electrodes are deployed in the form of a substantially spherical array. The electrode array is spatially referenced to a point on the endocardial surface by the reference electrode or by the reference catheter, which is brought into contact with the endocardial surface. Knowledge of the location of each of the electrode sites on the array, as well as a knowledge of the cardiac geometry is determined by impedance plethysmography.
U.S. Pat. No. 5,311,866 to Kagan et al., which is incorporated herein by reference, describes a heart mapping catheter assembly including an electrode array defining a number of electrode sites. The mapping catheter assembly has a lumen to accept a reference catheter having a distal tip electrode assembly which may be used to probe the heart wall. In the preferred construction, the mapping catheter includes a braid of insulated wires, preferably having 24 to 64 wires in the braid, each of which are used to form electrode sites. The catheter is said to be readily positionable in the heart to be used to acquire electrical activity information from a first set of non-contact electrode sites and/or a second set of in-contact electrode sites.
U.S. Pat. Nos. 5,385,146 and 5,450,846 to Goldreyer, which are incorporated herein by reference, describe a catheter that is said to be useful for mapping electrophysiological activity within the heart. The catheter body has a distal tip which is adapted for delivery of a stimulating pulse for pacing the heart or for ablating tissue in contact with the tip. The catheter further comprises at least one pair of orthogonal electrodes to generate a difference signal indicative of the local cardiac electrical activity adjacent the orthogonal electrodes.
U.S. Pat. No. 5,662,108 to Budd et al., which is incorporated herein by reference, describes a process for measuring electrophysiological data in a heart chamber. The method involves, in part, positioning a set of active and passive electrodes in the heart; supplying current to the active electrodes, thereby generating an electric field in the heart chamber; and measuring this electric field at the passive electrode sites. In one of the described embodiments, the passive electrodes are contained in an array positioned on an inflatable balloon of a balloon catheter.